A digital-to-analog converter is essential in presenting the discrete (usually binary) signal results of digital computation, storage, or transmission, into the form of the value or magnitude of some characteristic of the signal for graphical display, audio sound reproduction, or the control of devices that operate with continuously varying quantities. Often, a digital-to-analog converter has multiple electrical inputs representing a parallel binary number and an output in the form of a voltage or current
The analog output of a digital-to-analog converter is generally proportional to the product of the digital input value and a reference value. In many applications, the reference is fixed, and the output bears a fixed proportion to the digital input. Thus, the relationship of the analog output is generally linear. In other applications, the reference, as well as the digital input, can vary. A digital-to-analog converter that is used in these applications is thus often referred to as a multiplying digital-to-analog converter.
Digital-to-analog converters in the past tended to be lumped-component electronic devices produced individually and individually calibrated and tested. The readily accessible construction of such digital-to-analog converters allowed comprehensive external test equipment, often general purpose laboratory equipment, to be connected to various test points. The calibration and testing generally relied upon a skilled human operator to vary the necessary digital inputs to the digital-to-analog converter and to monitor the analog outputs. Consequently, the manufacturing costs were relatively high given the expense in test equipment and labor costs for testing
Digital-to-analog converters were later constructed using integrated circuit technology, such as fabricating a quad DAC dual in-line package (DIP) integrated circuit. Economic savings were realized by including most or all of the necessary electronic circuits for each digital-to-analog converter in a single semiconductor integrated circuit. Testing remained feasible since each digital-to-analog converter included external pins as test points for the digital input and analog output. Moreover, programmable external test equipment became available to generate the digital inputs and to monitor the analog outputs.
However, some applications of digital-to-analog converters in more recent devices are difficult to test. For example, digital-to-analog converters are used in the design of analog-to-digital converters that employ feedback techniques, such as successive-approximation and counter-comparator types. In such applications, the digital-to-analog converter may not necessarily appear as a separate identifiable entity. Moreover, the numbers of digital-to-analog converters incorporated into a single semiconductor integrated circuit may be large. Consequently, adding external connections for external test equipment is often impractical. Even if external connections would be feasible in some applications, an economic cost is incurred in using external test equipment, due to the cost of the equipment itself as well as delays incurred in the production line due to the testing.
Consequently, fabricating testing capability into each integrated circuit device that has a digital-to-analog converter would be beneficial. Unfortunately, known external testing equipment generally relies on comprehensive and complex testing algorithms. Integrated circuit designs replicating such comprehensive testing would undermine, if not render completely impractical, the goal of testing digital-to-analog converters in a more economic fashion. Moreover, in some applications, comprehensive testing of a digital-to-analog converter is not warranted. For example, the design and/or manufacturing process may tend to have very little variation in performance of the digital-to-analog converter. As another example, the digital-to-analog converter is used in an application requiring modest performance and only a basic functionality test is required.
Therefore, a significant need exists in the art for a simplified approach to functionality testing of digital-to-analog converters, and in particular, an approach that may be suitable for integration into a semiconductor integrated circuit containing one or more digital-to-analog converters.